1. Field of the Invention
The present invention relates to a semiconductor device, and more specifically to a semiconductor device in which a semiconductor substrate having a high breakdown voltage transistor formed thereon is covered with a sealing resin.
2. Description of the Background Art
Recently, as a power source IC used in a power source circuit, a power source IC of a switching system is used because it is compact, lightweight and highly efficient. As an element for controlling the power of such a power source IC, a power MOSFET (field effect transistor) is generally used because of the reasons of power consumption and miniaturization. A power MOSFET, which is produced based on a CMOS process, is also low-cost.
In order to drive a power MOSFET at high speed, the device including the power MOSFET needs to have a high breakdown voltage. For example, a power source IC of a switching system driven at AC100 to 200 V needs to have a breakdown voltage of 700 V or higher in order to drive a transistor. A semiconductor device including a power MOSFET having such a breakdown voltage characteristic is usually packaged with a resin when provided as a product. However, in a semiconductor device packaged with a resin, the drain breakdown voltage tends to be lowered as time passes when exposed to a high temperature and high humidity atmosphere. For suppressing the reduction in the drain breakdown voltage, various proposals have been made.
One reason that the drain breakdown voltage is reduced is that movable ions are accumulated at an interface between the passivation film of the semiconductor device and the sealing resin used for packaging the semiconductor device in a high temperature and high humidity atmosphere, and thus distort the equipotential distribution of the drain voltage and cause a local concentration of electric field. N. Fujishima, M. Saito, A. Kitamura,Y. Urano, G. Tada and Y. Tsuruta, “A 700 V Lateral Power MOSFET with Narrow Gap Double Metal Field Plates Realizing Low On-resistance and Long-term Stability of Performance”, Proceeding of International Symposium on Power Semiconductor Device & ICs, 2001, pp. 255-258, proposes a semiconductor device capable of shielding the influence of accumulated ions and thus alleviating the local concentration of electric field by narrowing the gap between the source electrode and the drain electrode. However, in this semiconductor device, a narrowed gap between the source electrode and the drain electrode strengthens the electric field on the surface of the substrate and thus lowers the breakdown voltage of the device. In order to suppress the reduction in the breakdown voltage of the device, the inter-layer film is made thicker than usual. For example, in order to narrow the gap between the source electrode and the drain electrode to 15 μm, the inter-layer film, which is usually about 1.5 μm thick, is made as thick as about 4.5 μm. However, a mere increase in the thickness of the inter-layer film is not sufficient to form a contact hole easily, and also enlarges the steps on the surface of the inter-layer film. In order to solve these problems, a two-layer structure is adopted for the source electrode and the drain electrode.
FIG. 13 is a cross-sectional view showing a structure of a lateral high breakdown voltage field effect transistor (MOSFET) including electrodes of a two-layer structure. In FIG. 13, the lateral high breakdown voltage MOSFET includes a P−-type substrate 310, an N+-type source region 320, an N−-type extended drain region 330, a gate insulating film 331, an N+-type drain region 340, a gate electrode 350, a first inter-layer film 360, a second inter-layer film 361, a first source electrode 370, a second source electrode 371, a first drain electrode 380, a second drain electrode 381, a passivation film 390, and a sealing resin 333.
The P−-type substrate 310 is a base substrate acting as a base for forming the MOSFET. In a main surface portion of the P−-type substrate 310, the N+-type source region 320, the N−-type extended drain region 330, and the N+-type drain region 340 are formed. The N−-type extended drain region 330 and N+-type drain region 340 are in contact with each other. The gate insulating film 331 is formed on a surface of the N−-type extended drain region 330, and an end of the gate insulating film 331 is extended to overlap the N+-type source region 320. The gate electrode 350 is formed on the gate insulating film 331. The first inter-layer film 360 is an insulating film formed on the gate insulating film 331 so as to cover the gate electrode 350.
A source electrode has a two-layer structure including the first source electrode 370 and the second source electrode 371. The first source electrode 370 is formed on the first inter-layer film 360 so as to be connected to the N+-type source region 320. The second source electrode 371 is formed so as to be connected to the first source electrode 370. Similarly, a drain electrode has a two-layer structure including the first drain electrode 380 and the second drain electrode 381. The first drain electrode 380 is formed on the first inter-layer film 360 so as to be connected to the N+-type drain region 340. The second drain electrode 381 is formed so as to be connected to the first drain electrode 380. The second inter-layer film 361 is formed between the first source electrode 370 and the second source electrode 371 and between the first drain electrode 380 and the second drain electrode 381. In this specification, the various elements formed on the P−-type substrate or an equivalent thereto will be collectively referred to a “semiconductor substrate body”. A surface of the substrate body including the electrodes of the two-layer structure is covered with the passivation film 390 formed of an SiN film. The passivation film 390 is covered with the sealing resin 333. The P−-type substrate 310 is electrically connected to the source in an area which is not shown in FIG. 13.
In the lateral high breakdown voltage MOSFET having the above-described structure, when a high voltage is applied to the second drain electrode 381 in an off state, a reverse voltage is applied to a junction between the N−-type extended drain region 330 and the P−-type substrate 310, and a depletion layer expands two-dimensionally in the longitudinal direction and the lateral direction in the N−-type extended drain region 330. As a result, the N−-type extended drain region 330 is completely depleted, and equipotential lines of the drain voltage are uniformly distributed in the N−-type extended drain region 330.
In a high temperature and high humidity atmosphere, movable ions indicated as anions 344 and cations 355 in FIG. 13 are accumulated at an interface of the sealing resin 333 with the passivation film 390. When the movable ions influence the N−-type extended drain region 330, the above-mentioned distribution of the equipotential lines of the drain voltage tend to be distorted to cause a local concentration of electric field, resulting in reduction in the drain breakdown voltage. In the above-described lateral high breakdown MOSFET including the electrodes of the two-layer structure, the N−-type extended drain region 330 is unlikely to be influenced by the movable ions accumulated at the interface between the passivation film 390 and the sealing resin 333 owing to the above-mentioned narrowed gap between the source electrode and the drain electrode. Therefore, the reduction in the drain breakdown voltage can be suppressed.
However, the production of the above-described lateral MOSFET including the electrodes of the two-layer structure requires a plurality of additional steps in addition to the general steps for producing a lateral MOSFET as follows: (i) the step of forming the second inter-layer film 361, (ii) the step of forming a contact hole by etching the second inter-layer film 361; (iii) the step of forming the second source electrode 371 and the second drain electrode 381, and (iv) the step of processing the second source electrode 371 and the second drain electrode 381. For the processing in steps (ii) and (iv), at least two additional photomasks are required. As can be seen from this, the lateral MOSFET including the electrodes of the two-layer structure has the problems of the complex production process and being highly costly. In addition, the two-layer structure of the electrodes is against the miniaturization of devices, which is being promoted in the field of power MOSFETs as in the other fields.